Here is my essential point. If Sheridan is a better commander that his subordinates, then is it not reasonable to assume that when the attack does take place his responses will be better and the attack less successful than in actual fact.Interesting question.
I, however, see little evidence that Sheridan's absence effected the outcome of the morning attack. He knew that Early was back at Fisher's Hill, however, he was not worried about Early attacking enough for him to cancel his trip to Washington on October 15th. Therefore, I think Sheridan would have been as surprised as Wright on the morning of the 19th. Further, I see no evidence that Early was aware of Sheridan's absence; so I do not see how that could have effected his planning.
I still agree with @JSylvester that Early's plan up to the Fatal Halt was "incomparably brilliant," and I believe Wright's response was excellent considering the circumstances. He coordinated as good a defense as can be expected, bravely led troops into battle himself despite being wounded, and even started regrouping his troops north of Middletown before Sheridan arrived back on the field. If anything, I think Sheridan's trip to Washington was a stroke of luck. I do believe his timely arrival back to the battlefield on the 19th did help rally and reinvigorate his army and if he had not gone to Washington that would not have happened.
I also agree with @7thWisconsin about Sheridan; he did understand war. However, I believe Sheridan made a mistake going to Washington on the 15th. After "The Burning," there was a flurry of communications between Grant, the leaders in Washington, and Sheridan. Grant had requested that Sheridan return part of his army to Petersburg. On the 10th Sheridan moved his army north, down the Valley through Strasburg toward Middletown. He set the 8th and 19th Corps into camp along the high ground above the Cedar Creek and ordered the 6th Corps to the Manassas Gap railroad to start their return to Petersburg. Tom's Brook happened that day so he figured only weak Cavalry was following him.
The morning of the 13th though, he received communications that Grant wanted him to threaten the Virginia Central Railroad and he should move towards Charlottesville, VA. Believing he would need the 6th Corps for that, he sent an order to Wright directing him to return. He still didn't know that Early had followed him north and had occupied Fisher's Hill and sent Gordon to Hupp's Hill on the 12th. Gordon attacked on the 13th though and Thoburn's 8th Corps Division engaged them in the battle of Hupp's Hill. Sheridan should have known then not to go. On the 15th Custer and Merritt's Cavalry and Emory's 19th Corps Divisions all engaged in reconnaissance and confirmed that Early was at Fisher's Hill. Despite this, he still decided to head to Washington. Then on the 16th Early had his signalmen flag a fake message indicating Longstreet was reinforcing Early. Wright intercepted it and sent that to Sheridan who was at Front Royal by then. Yet, instead of returning to take command of his army, he messaged with Washington who told him Longstreet was still on Petersburg, but Kershaw might be gone. Halleck told Sheridan he didn't need to come to Washington, but all Sheridan did was sent Merritt back to reinforce Wright and continue on.
I think "what if's" can be fun, but we will never really know. I think all this evidence supports an argument that Sheridan would have been just as surprised as Wright, if not more so (Wright was more concerned about the fake signal than Sheridan was), and that the Union response would not have been "quicker and more effective" had he been on the field. It seems clear to me that he was not worried that Early was a serious threat, therefore I believe the morning probably would have gone pretty much just as it had under Wright. And without the added effect of him riding down and rallying fleeing troops, I think the day could have gone worse had he been there? I think Sheridan should be criticized for leaving for Washington on the 15th with overwhelming evidence that Early was near, being aggressive (Hupp's Hill), and probably being reinforced by a Division at the least.
In actual fact Sheridan returns and turns abject defeat into victory. If he is there from the beginning does he turn temporary Southern advances into a complete disaster. Remember he took charge of a defeated and totally disorganized army and rallied it to return to battle and defeat the opponent. How much better could he have done if he had been on hand from the initial onslaught?